As the size of MOS transistors is scaled down, the breakdown voltage thereof is lowered as stated, for example, in the 1989 International Symposium on VLSI Technology, Systems and Applications, Proceedings of Technical Papers, pp. 188-192 (May 1989). Accordingly, the operating voltage thereof has to be lowered. In particular, the operating voltage is lowered even more for the purpose of achieving low power consumption for semiconductor devices used in a battery-operated portable equipment and the like.